1. Field of the Invention
The invention relates to methods and apparatus for cleaning and passivating laser discharge chambers. More particularly, the invention relates to methods and apparatus for cleaning and passivating laser discharge chambers utilizing plasmas.
2. Description of the Related Art
Gas lasers in which the lasing medium includes fluorine or fluorine compounds are the workhorse light sources for the integrated circuit lithography industry. In fluorine based gas lasers such as krypton fluoride (KrF) excimer lasers, argon fluoride (ArF) excimer lasers, and molecular fluorine (F2) lasers, a high energy electrical discharge excites a gas mixture in a discharge chamber to produce a plasma which serves as the lasing medium.
The performance of such lasers is degraded by the presence of impurities in the discharge chamber inadvertently introduced as contaminants during the manufacturing process, introduced by exposure to the ambient environment, or produced by reactions between the gas mixture and contaminants or chamber materials. Such impurities include HF, CF4, COF2, SiF4, CO2, various hydrocarbons, and H2O. Impurities can degrade the profile of the laser beam by fouling optical components, reduce the lifetime of the gas fill by reacting with and consuming the gas mixture, and reduce output power by absorbing laser light and by quenching the excited species such as ArF, KrF, and F2 that support lasing. Also, highly reactive impurities such as HF corrode the internal surfaces of the discharge chamber. Impurities are detrimental even at low concentrations. For example, it has been observed that the presence of CO2 at concentrations as low as 30 parts per million in a KrF excimer laser plasma can reduce the output power of the laser by 5%. Consequently, the discharge chamber must be cleaned of impurities.
The plasma lasing medium includes highly reactive fluorine species which also corrode unprotected internal surfaces of the discharge chamber. Consequently, the materials in the discharge chamber must be passivated to protect them from the plasma lasing medium.
Laser discharge chambers for fluorine based gas lasers are conventionally cleaned and passivated with a thermal process such as the following. A discharge chamber is heated to approximately 100xc2x0 C. and evacuated with a vacuum pump to a pressure of approximately 20 millitorr. This temperature and pressure is maintained for at least 8 hours, during which some of the volatile contaminants in the discharge chamber, such as water, are removed by the pump. The discharge chamber is then filled with a mixture of approximately 5% F2 and approximately 95% helium, neon, or other inert gas at a pressure of approximately one atmosphere. The temperature is maintained at 100xc2x0 C. for at least 4 hours, during which a fluorine based passivation layer forms on some of the internal surfaces of the discharge chamber.
The fluorine gas used in conventional processes poses a safety risk, as it is highly corrosive and highly toxic. Also, the conventional process is not entirely effective. The discharge chamber must undergo a subsequent 24 hour bum-in operation period, during which the gas mixture is replaced multiple times, before laser operation is satisfactory. Furthermore, the conventional process requires at least 12 hours, more typically 24 to 48 hours, and is therefore inefficient.
What is needed is a laser discharge chamber cleaning and passivation process that is safer, more effective, and more efficient than conventional cleaning and passivation methods.
Methods and apparatus are provided for cleaning and passivating laser discharge chambers with plasmas. In one embodiment, an oxygen based plasma is formed in an external plasma source by inductively applying a radiofrequency signal to oxygen containing gases such as O2, N2O, and mixtures thereof. The oxygen based plasma is drawn into the laser discharge chamber, where it reacts with contaminants to produce volatile species which are removed by a vacuum pump, thereby cleaning the laser discharge chamber.
After the oxygen based plasma cleaning process, a fluorine based plasma is formed in the external plasma source by inductively exciting fluorine containing gases such as NF3, F2, CF4, SF6, and mixtures thereof with a radiofrequency signal. The fluorine based plasma is drawn into the laser discharge chamber, where it reacts with internal surfaces to form a protective passivation layer. The fluorine based plasma also reacts with contaminants to produce volatile species which are removed by the vacuum pump. Internal surfaces of the laser discharge chamber are thereby cleaned and passivated. Since the fluorine based plasma also cleans the chambers, the oxygen based plasma cleaning process is optional.
In another embodiment, oxygen based plasma and fluorine based plasmas are formed in the laser discharge chamber by applying a radiofrequency signal to a laser discharge chamber electrode and thereby exciting oxygen containing gases and fluorine containing gases. The oxygen and fluorine based plasmas react with contaminants and with internal surfaces to clean and passivate the laser discharge chamber.
Plasma cleaning and passivation of laser discharge chambers does not require the use of dangerous F2 gas. Also, plasma cleaning and passivation is much less time consuming, and thus much more efficient, than conventional thermal cleaning and passivation processes. Moreover, laser discharge chambers cleaned and passivated with plasmas exhibit improved performance.